The invention relates to screws and to plates for use in osteosynthesis.
Osteological screws, as well as osteological pins having a threaded portion for bones, are mostly used in combination with plate and rod systems to fix bones and bone segments in specific position and alignment relative to on another.
During conventional osteosynthesis on tubular bones, a plate provided with holes is fixed to the bone by means of osteological screws which pass through these holes. Immobilization of the bone or bone segments is carried out by pressing the same against the plate via screws. The screw holes are countersunk on the side opposite the bone where they constitute an abutment for the heads of the screws. Stable connection of the bones or bone segments is thus achieved by pressing of the screw heads against the osteological plate and the resulting secondary pressing of the osteological plate against the bone itself. The osteological screws anchored in the bone are subjected to practically pure tension within the bone itself since the bending forces in this area are absorbed by the bone tissue. Much greater mechanical forces, especially bending loads, arise at the exit point of the screws from the bone. The transition of the osteological thread of conventional screws accordingly constitutes a stress concentration. The material is additionally weakened by a recess which is intended for a tool and projects into the neck of the screw.
In clinical practice, therefore, screw fractures are constantly seen and typically occur at these weakened locations. Inasmuch as the broken portion of the screw remaining in the bone is practically flush with the surface of the bone and does not present a gripping area, relatively large bores, which weaken the bone considerably, must be formed in the bone by means of hollow drills in order to remove the broken portion.
In setting procedures using threaded osteological pins which are fixed to rod systems via a lateral connection, such fractures occur somewhat less frequently because of the greater elasticity and the lower stress concentration existing with large contact areas. Nevertheless, the longer lever arm and the greater distance between the exit point from the bone and the fixing location at the rod system leads to bending of the pins and, consequently, to inadequate fixing of the bone or bone segments. The results are undesired changes in position and failure of the bone to heal.
In procedures involving the application of plates to the spinal column, stressing of the osteological screws at the transition to the screw head is still greater since the anatomical characteristics allow each bone to be fixed to the osteological plate by a single osteological screw only. Stability is further reduced because the osteological plates cannot be pressed against the bone contact surfaces over a large area to thereby compensate for part of the bending load by tension/compression stressing. Since, on the one hand, substantial forces and loads arise (virtually the entire body weight bears on a fixing screw) while, on the other hand, pressure between plate and bone and, secondarily, between plate and screw, is greatly reduced, the immobilization required for bone consolidation frequently cannot be achieved. Due to the point overloading of the screws in conjunction with the increased bending load, they loosen or fracture much more often than in osteosynthesis at the extremities. Loosening, in turn, is caused by the fact that solid anchoring occurs only in the region of contact of the osteological screw with cortical structures ( rind of the bone). Whereas the screws are always anchored in two bone rinds with the tubular bones of the extremities, the screws are screwed through the narrow, bony connections between the anterior vertebral bodies and the lateral vertebral arches during setting of the spine. These bony bridges, referred to as arch roots (pedicels), have the form of a yarn spool and it is exclusively in the middle, i.e., narrow, portion that good, direct force transmission to the centrally extending osteological screw, which only here tangentially contacts the rind of the bone, can be achieved. Substantial force transmission is hardly possible dorsally of this isthmus and in particular, there is no bone rind to support the osteological screw at its exit point.
Stable setting of the spinal column is difficult to begin with and possible only when the thread of the osteological screw directly contacts the pedicel-corticalis in the region of the isthmus. The stress concentration, which is naturally a function of how solidly the screw is anchored in the bone, occurs either at the heat of the screw or, more often, several millimeters from the head of the screw where the screw loses contact with the isthmus-corticalis. These concentrations are highly likely to cause screw fratures in the posterior pedicel region in about 10% of clinical applications, especially when there is angularly stable anchoring of the osteological screws to an osteological plate. The fractures occur approximately 2 to 8 mm before the exit point of the screw from the pedicel, i.e., in the latter. Due to the close proximity of the spinal cord, such complications are particularly problematical and difficult to control.